Bob C
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| posted on 15/12/05 at 10:42 AM |
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Rear steer (not locost)
I'm involved with a local school electric car racing team & looking at designs for next years car. Aerodynamics may make it convenient for
us to have the steering on the rear wheels.
I'm worried about 'self centring' in that I suspect it is practically impossible (it looks like a "non- minimum phase"
control system to me) so zero castor and springs should be employed. Any thoughts or experiences out there?
thanks
Bob
[Edited on 15/12/05 by Bob C]
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JonBowden
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| posted on 15/12/05 at 10:56 AM |
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Some old light aircraft do this. Apparently they are dificult to control on the ground though
Jon
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NS Dev
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| posted on 15/12/05 at 11:11 AM |
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most forklifts do it.
don't know the relevance though!
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Bob C
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| posted on 15/12/05 at 11:47 AM |
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Forklifts - what are you like- this has to be driven by 14yr old schoolgirls at 40mph!!!!
If a 'plane is at any sort of speed I guess the tailfin does most of the work - and that has a big non-steering part to straighten things
up!
So do forklifts have any castor built in???
specific problem I see is that with conventional castor, it works nicely to resist rotational acceleration by reacting against the polar inertia of
the vehicle - but on a long bend the centrifugal effect is trying to increase the steering. Put the castor the other way & you get high frequency
instability due to the inertia thing. You're in the brown stuff either way!
Bob
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MikeR
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| posted on 15/12/05 at 12:07 PM |
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ok, can't remeber my castor / camber without a diagram (Ask NS Dev, talking about his rear steer idea whilst driving last night and the
conversation was filled with "the wheel leaning in" phrases).
think about a shopping trolley castor wheel. Something like that, with a bit of toe out and i think you'll be fine.
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dave dickson
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| posted on 15/12/05 at 06:59 PM |
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One of the wee tractors at work is rear-wheel steering. Its top speed is maybe a fast running speed.
It has absolutely no self-centering, and is VERY twitchy at speed. Its a bit of work to keep it in a straight line.
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kb58
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| posted on 15/12/05 at 08:31 PM |
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wouldn't adding caster at the back work the same as at the front, to give a return-to-center force? It's the opposite case of driving a
car in reverse, the caster at the front makes it very hard to control, but that's because it's going in reverse. Putting in opposite
caster should solve the instability... I think.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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jambojeef
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| posted on 15/12/05 at 11:06 PM |
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Hi bob,
The Greenpower thing eh! We're trundling along with ours at the minute too!
Wish we were advanced enough to be worrying about aerodynamics!
Geoff
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Bob C
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| posted on 16/12/05 at 12:57 AM |
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Hi Kurt,
I'm assuming the difference would be that with front steer & normal (forward ) castor the centrifugal effect is trying to straighten the
steering, but with rear steer & forward caster it's trying to wind on more lock. It's all just theory though! It sounds as though a
few folk have experience that rear steer is 'scary'.
Yeh Goeff, we're trying hard with aero this year. 'turbo tortoise' (winner) is clearly showing us all the way! If we get it right we
can beat it 'cos we're better in some other departments...
Cheers
Bob
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iank
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| posted on 16/12/05 at 01:12 AM |
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My memory for obscure facts told me that Thrust SSC had rear steer, so with the help of google I've found:
Some general information about how they designed it
http://www.thrustssc.com/thrustssc/Driver/Testing_the_Mini.html
and the strange world of the experimental bike designer 
http://wannee.nl/hpv/abt/e-index.htm
Hope it helps - too late to delve too deeply.
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akumabito
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| posted on 16/12/05 at 01:12 AM |
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I can imagine it would be scary to drive a rear-steered vehicle at over 3MPH! The car would do just about the exact opposite of what you'd
expect it to do. The front would not follow nice and redictable lines, but instead the back would just swerve out "pushing" the front to
the side... If you want a good demonstration of what this means: get a normal car up to around 20 MPH, then turn the wheel as fast as you can and see
what happens. Next, do the same, only driving in reverse. Hope your insurance covers crazy driving behavior, because you're very likely to roll
the vehicle over... :S
Rear-wheel steering really is a horrible idea. Think about it, over the past century, the world has seen hundreds of car manufacturers, and thousands,
if not tens of thousands of different models of all kinds of types. And not a single production vehicle EVER has used rear-wheel steering. There might
be a very good reason for that, don't you think?
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MikeR
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| posted on 16/12/05 at 08:35 AM |
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no one thought of it ?
(he might have a point)
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iank
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| posted on 16/12/05 at 10:14 AM |
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quote:
Originally posted by akumabito
I can imagine it would be scary to drive a rear-steered vehicle at over 3MPH! The car would do just about the exact opposite of what you'd
expect it to do. The front would not follow nice and redictable lines, but instead the back would just swerve out "pushing" the front to
the side... If you want a good demonstration of what this means: get a normal car up to around 20 MPH, then turn the wheel as fast as you can and see
what happens. Next, do the same, only driving in reverse. Hope your insurance covers crazy driving behavior, because you're very likely to roll
the vehicle over... :S
Rear-wheel steering really is a horrible idea. Think about it, over the past century, the world has seen hundreds of car manufacturers, and thousands,
if not tens of thousands of different models of all kinds of types. And not a single production vehicle EVER has used rear-wheel steering. There might
be a very good reason for that, don't you think?
Well the reverse steering direction can be engineered out pretty trivially, and the fastest 'car' ever used it so it must be the way
forward 
Cost, marketing, timing and inertia are all reasons why the 'best' solution may not win in the market.
VHS vs. Beta
Qwerty vs. Dvorak
Microsoft windows 3.1 vs OS/2
Once established it's very hard to introduce anything radical unless it works in exactly the same way.
However, I agree rear steer probably does have one major drawback (from the limited reading I've done) it seems (by default) to do the reverse
of self center, so a small input uncorrected produces yet more steering - i.e. positive feedback. That's dangerous on a road car because you
have to concentrate and correct all the time.
I think it can probably be dialed out with camber in the opposite way a lot of locosts can be made to self steer properly given better wishbone
geometry. But I'm not convinced you will ever get better than neutral steering (at least not without computer steer by wire assistance).
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Liam
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| posted on 16/12/05 at 03:01 PM |
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quote:
Originally posted by akumabito
I can imagine it would be scary to drive a rear-steered vehicle at over 3MPH! The car would do just about the exact opposite of what you'd
expect it to do. The front would not follow nice and redictable lines, but instead the back would just swerve out "pushing" the front to
the side... If you want a good demonstration of what this means: get a normal car up to around 20 MPH, then turn the wheel as fast as you can and see
what happens. Next, do the same, only driving in reverse. Hope your insurance covers crazy driving behavior, because you're very likely to roll
the vehicle over... :S
The reason a normal car is horrible to steer if driven in reverse is because going backwards you have negative caster on the steering wheels i.e.
anti-self centering. If a car was properly designed to be rear steer with positive caster on the rear wheels it wouldn't be so bad.
Liam
[Edited on 16/12/05 by Liam]
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Bob C
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| posted on 16/12/05 at 03:41 PM |
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Thanks iank - the 'wannee' site has lots of info (none of it encouraging) - seems like all the systems tried, as you go faster it
eventually becomes unstable. Another factor I hadn't considered is that you can't drive away from walls!!!! Mind you I noticed no
prototype centrpoint zero castor designs. . . .
cheers
Bob
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MikeR
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| posted on 16/12/05 at 05:00 PM |
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you can drive away from walls, you just either have to be in a tank and demolish them or put the car in 'reverse'
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MikeRJ
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| posted on 17/12/05 at 11:08 PM |
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quote:
Andy Green, driver of Thrust SSC
There was only one problem - I couldn’t make the rear wheels skid! Even at the limit, with the front wheels sliding sideways in a turn or locked under
braking, the rear wheels gave positive control all the time, leaving me nothing to investigate!
Someone forgot to install the handbrake methinks!
[Edited on 17/12/05 by MikeRJ]
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